1. Field of the Invention
The present invention relates generally to living olefin polymerization processes, and more specifically to initiators for such processes that are stable under reaction conditions in the absence of olefin monomer such that polymers of low polydispersity can be synthesized.
2. Discussion of the Related Art
Polymers are used in a large number of applications, and a great deal of attention has been paid to developing synthetic routes that result in polymers having optimal physical and chemical properties for a given application.
Block copolymers are one class of polymers that have broad utility. For example, block copolymers have been employed as melt processable rubbers, impact resistant thermoplastics and emulsifiers. As a result, these materials have been the focus of a particularly large amount of research and development both in industry and academia, and a variety of approaches to block copolymer synthesis have been developed.
When preparing a block copolymer, it is generally desirable to use a synthetic technique that allows for control over the chain length of each polymer block and the polydispersity of the resulting block copolymer. For some time, attempts to provide such a method have focused on block copolymer formation by living polymer synthesis. In living polymer synthesis, a metal-containing initiator having either a metal-carbon bond or a metal-hydrogen bond is reacted with an olefin monomer to form a polymer chain via the successive insertion of the first olefin monomer into a metal-carbon bond between the metal of the initiator and the growing polymer chain. If the initiator is a metal-hydride complex, the first metal-carbon bond is formed when the olefin inserts into the metal-hydride bond. When the olefin monomer is depleted, a second olefin monomer is added, and a second polymer block is formed by successively inserting, into the metal-carbon end group, the second monomer, ultimately resulting in a block copolymer including a first polymer block connected to a second polymer block. Since each polymer block is formed sequentially, the initiator and propagating species should be stable under reaction conditions in the absence of olefin monomer.
To provide a block copolymer having sizable polymer blocks of low polydispersity, the rate of chain propagation (i.e., olefin monomer insertion into the metal-carbon bond) should be substantially greater than the rate of chain termination or transfer. To prepare a block copolymer having the lowest possible polydispersity, the rate of initiation should be at least as great as the rate of propagation.
Polymerization termination is typically dominated by .beta.-hydride elimination with the products being a polymer chain having a terminal carbon-carbon double bond and the initiator having a metal-hydrogen bond. Termination of polymerization also can occur if the initiator decomposes in some other manner, such as transfer of the polymer chain from the initiator to some other element that is relatively inactive in or for olefin polymerization. Hence, the achievable chain length of copolymer blocks and the polydispersity of the block copolymer are principally determined by the relative rates of olefin insertion and .beta.-hydride elimination, as well as initiator stability toward other modes of decomposition, especially in the absence of olefin monomer.
Attempts at synthesizing polymers using living polymer synthesis have employed a variety of initiators. For example, as reported in JACS 118, 10008 (1996), McConville and co-workers have used a diamido-titanium initiator to form polymers by polymerizing .alpha.-olefins. In addition, Turner and co-workers have developed a hafnium-containing cyclopentadienyl initiator for preparing block copolymers from .alpha.-olefin monomers (published PCT patent application WO 91/12285). Furthermore, Horton and co-workers report diamido-group IVB metal initiator effective in providing homopolymer synthesis (Organometallics 15, 2672 (1996)).
Despite the commercial motivation for developing a living polymer synthetic method for block copolymer preparation, known methods of block copolymer synthesis can suffer from a variety of problems. For example, the initiators used can be unstable under reaction conditions in the absence of olefin monomer, resulting in an inability to form additional homopolymer blocks to form a block copolymer. Moreover, the efficiency of block copolymer formation can be reduced due to the formation of significant amounts of homopolymer. In addition, due to the low temperatures used, the products formed using many known initiators have relatively low molecular weights and are more appropriately classified as oligomers.
As seen from the foregoing discussion, it remains a challenge in the art to provide a method of synthesizing block copolymers that includes the use of a initiator that is stable in the absence of olefin monomer such that the resulting block copolymers have low polydispersities. Such an initiator would also offer the advantage of resulting in relatively small amounts of homopolymer synthesis.